The present invention relates to a method of forming a metallic product, and more particularly to a method of forming an engine valve.
There are known various methods of forming engine valves for use as intake and exhaust valves. According to one conventional method, a rod-shaped blank, which is larger in diameter than the stem of a valve to be formed, is heated and then extruded, except for one end thereof, into a preliminary product having a reduced diameter that is the same as the diameter of the valve stem. Thereafter, the larger-diameter end, which was not extruded, of the preliminary product is formed into a valve head, and any burrs on the valve head are removed by trimming, thereby forming the valve. The above valve forming method is referred to as an "extrusion process".
Another prior method uses an electric upsetter which has an anvil electrode and a crank electrode for sandwiching a rod-shaped blank having substantially the same diameter as that of the stem of a valve to be formed. When the rod-shaped blank is sandwiched between the anvil and crank electrodes, an electric current is passed through the blank between the electrodes while at the same time the blank is being pressed thereby to form a preliminary product with a thickened end having a larger diameter. The thickened end is then formed into a valve head and deburred by trimming, producing the valve. This method of forming a valve is referred to as an "electric upsetting process".
To meet a growing demand for hither engine power output and performance, various efforts are being made to make engines lighter and improve intake and exhaust efficiencies. One attempt is directed to reducing the diameter of valve stems and increasing the diameter of valve heads, i.e., increasing the ratio of the diameter of valve heads to the diameter of valve stems. Such an attempt necessitates an increase in the mechanical strength and heat resistance of valves. In view of this, some valves are made of heat-resistant superalloys.
The desired valve configurations and materials require that the following conditions be satisfied in the formation of valves:
First, since heat-resistant superalloys used as valve materials are difficult to form, they are subjected to less plastic deformation. For forming a valve head, it is necessary that the larger-diameter or thickened end of a preliminary product be reliably shaped as desired with its diameter close to that of the desired valve head. Secondly, because heat-resistant superalloys are generally expensive, it is preferable to form valve heads by closed-die forging so that no burrs will be produced and hence no trimming is required, with the result that valves can be produced with an increased yield.
According to the extrusion process, when a preliminary product is formed, a rod-shaped blank tends to be easily cooled by contact with a die. This fact, together with the difficult formability of heat-resistant superalloys, prevents the diameter of the rod-shaped blank from being greatly reduced. In order to produce a valve with a thin stem, it is necessary to employ a small-diameter rod-shaped blank. If a small-diameter rod-shaped blank is employed, however, then the larger-diameter end of a preliminary product cannot be enlarged to the desired diameter of the head of a valve to be formed. For increasing the diameter of the head of a valve, the larger-diameter end of the preliminary product has to be greatly enlarged when the valve head is formed. At this time, the formed valve head is liable to crack. When the valve head is to be formed, inasmuch as the larger-diameter end of the preliminary product has to be deformed to a large extent, the plastic deformation thereof must be effected smoothly. However, such smooth plastic deformation cannot be achieved by closed-die forging.
As described above, the rod-shaped blank is cooled by the die when it is formed into the preliminary product. The preliminary product should therefore be reheated before its larger-diameter end is shaped into a valve head. The reheating step necessarily increases the number of steps required to produce valves and lowers the production efficiency.
According to the electric upsetting process, the thickened end of a preliminary product is formed by upsetting one end of the preliminary product while heating the end. Therefore, the electric upsetting process makes it possible to allow the thickened end to have a relatively large diameter as compared with the stem of the preliminary product. The preliminary product is not required to be reheated before its thickened end is formed into a valve head. The electric upsetting process is therefore more suitable than the extrusion process for increasing the ratio of the diameter of the head to the diameter of the stem of a valve. However, the rod-shaped blank is required to be small in diameter in order that the stem of a formed valve will be thin. With the electric upsetting process, it is generally difficult to form the preliminary product into a desired shape from which a valve stem can reliably be obtained.
With the electric upsetting process, since the shape of a preliminary product is not confined or forcibly defined by a die or the like, when a rod-shaped blank is reduced in diameter, it is apt to have some defects such as cracking, buckling, material localization, or wrinkling. In order to eliminate such deficiencies and reliably form a preliminary product of desired shape, it is necessary to accurately control the speed at which one end, to be formed into a thickened end, of a rod-shaped blank, is displaced or pushed toward the anvil electrode of an electric upsetter and also the temperature to which the rod-shaped blank is to be heated, depending on the material of the rod-shaped blank.
Japanese Laid-Open Patent Publication No. 60(1985) -127037 discloses a mechanical control system for controlling the pushing speed at which a rod-shaped blank is pushed toward an anvil electrode. The disclosed mechanical control system employs a cylinder for pushing or displacing the rod-shaped blank, and the pushing speed is mechanically controlled by the control of a pushing force produced by the cylinder. However, the pushing speed cannot accurately be controlled by the disclosed mechanical control system.
The anvil electrode of an electric upsetter is normally made of a highly electrically conductive material such as a copper alloy or the like. The anvil electrode of such a material is also highly thermally conductive, the heat of a rod-shaped blank which is pressed against the anvil electrode is transferred away from the rod-shaped blank through the anvil electrode, making the temperature of the rod-shaped blank unstable. It has been difficult to achieve stable control of the temperature of the heated rod-shaped blank.
Even if the pushing speed is controlled in the conventional electric upsetting process, therefore, various problems still remain to be solved when preliminary products are formed, and the formed preliminary products tend to have different shapes. If the formed preliminary products are not uniform in their shape, then when valve heads are formed from the preliminary products, necks positioned between the valve heads and stems of valves are liable to wrinkle. The inventors have found that such wrinkles on the necks are more appreciably produced when the valve heads are formed by closed-die forging.
The inventors have made various efforts to analyze the problems described above. As a result of the study, the inventors have found that the pushing speed at which a rod-shaped blank is pushed toward an anvil electrode can accurately be controlled by controlling a current supplied to the rod-shaped blank, insofar as the pushing force applied to push the rod-shaped blank toward the anvil electrode is constant.
The inventors have also found that the temperature of an anvil electrode can stably be maintained if it is made of a nicked- or cobalt-base heat-resistant superalloy which is less thermally conductive than copper alloys.
The inventors have also concentrated their efforts to find out why wrinkles tend to be created on a valve neck when a valve head is formed by closed-die forging. As a result, it has been discovered that there is a close relationship between the angle of inclination from the stem toward the thickened portion of a preliminary product (hereinafter referred to as a "thickened-portion angle") and the angle of inclination of a forming surface of a die which forms an inclined surface of the valve neck (hereinafter referred to as a "forming-surface angle").
More specifically, if the thickened-portion angle is larger than the forming-surface angle, then there is created a relatively large gap or clearance between the preliminary product and the forming surface of the die, which are therefore not held in close contact with each other. When the valve head is formed, the material of the preliminary product near the forming surface is likely to be easily displaced inwardly, resulting in wrinkles on the valve neck. This defect manifests itself since the thickened end of the preliminary product is less subjected to plastic deformation in closed-die forging.